Frueh B R, Gregorevic P, Williams D A, Lynch G S
Department of Physiology, The University of Melbourne, Victoria 3010, Australia.
J Neurophysiol. 2001 Mar;85(3):1027-32. doi: 10.1152/jn.2001.85.3.1027.
Extraocular muscles are characterized by their faster rates of contraction and their higher resistance to fatigue relative to limb skeletal muscles. Another often reported characteristic of extraocular muscles is that they generate lower specific forces (sP(o), force per muscle cross-sectional area, kN/m(2)) than limb skeletal muscles. To investigate this perplexing issue, the isometric contractile properties of the levator palpebrae superioris (levator) and superior rectus muscles of the rat were examined in situ with nerve and blood supply intact. The extraocular muscles were attached to a force transducer, and the cranial nerves exposed for direct stimulation. After determination of optimal muscle length (L(o)) and stimulation voltage, a full frequency-force relationship was established for each muscle. Maximum isometric tetanic force (P(o)) for the levator and superior rectus muscles was 177 +/- 13 and 280 +/- 10 mN (mean +/- SE), respectively. For the calculation of specific force, a number of rat levator and superior rectus muscles were stored in a 20% nitric acid-based solution to isolate individual muscle fibers. Muscle fiber lengths (L(f)) were expressed as a percentage of overall muscle length, allowing a mean L(f) to L(o) ratio to be used in the estimation of muscle cross-sectional area. Mean L(f):L(o) was determined to be 0.38 for the levator muscle and 0.45 for the superior rectus muscle. The sP(o) for the rat levator and superior rectus muscles measured in situ was 275 and 280 kN/m(2), respectively. These values are within the range of sP(o) values commonly reported for rat skeletal muscles. Furthermore P(o) and sP(o) for the rat levator and superior rectus muscles measured in situ were significantly higher (P < 0.001) than P(o) and sP(o) for these muscles measured in vitro. The results indicate that the force output of intact extraocular muscles differs greatly depending on the mode of testing. Although in vitro evaluation of extraocular muscle contractility will continue to reveal important information about this group of understudied muscles, the lower sP(o) values of these preparations should be recognized as being significantly less than their true potential. We conclude that extraocular muscles are not intrinsically weaker than skeletal muscles.
眼外肌的特点是与肢体骨骼肌相比，其收缩速度更快，对疲劳的抵抗力更强。眼外肌另一个经常被报道的特点是，它们产生的比肌力（sP(o)，每肌肉横截面积的力，kN/m²）低于肢体骨骼肌。为了研究这个令人困惑的问题，在原位完整保留神经和血液供应的情况下，对大鼠提上睑肌和上直肌的等长收缩特性进行了检查。将眼外肌连接到力传感器上，并暴露颅神经进行直接刺激。在确定最佳肌肉长度（L(o)）和刺激电压后，为每块肌肉建立了完整的频率-力关系。提上睑肌和上直肌的最大等长强直力（P(o)）分别为177±13和280±10 mN（平均值±标准误）。为了计算比肌力，将一些大鼠提上睑肌和上直肌保存在20%的硝酸溶液中以分离单个肌纤维。肌纤维长度（L(f)）表示为总肌肉长度的百分比，从而可以使用平均L(f)与L(o)的比值来估计肌肉横截面积。提上睑肌的平均L(f):L(o)为0.38，上直肌为0.45。在原位测量的大鼠提上睑肌和上直肌的sP(o)分别为275和280 kN/m²。这些值在通常报道的大鼠骨骼肌sP(o)值范围内。此外，在原位测量的大鼠提上睑肌和上直肌的P(o)和sP(o)显著高于（P < 0.001）在体外测量的这些肌肉的P(o)和sP(o)。结果表明，完整眼外肌的力输出根据测试模式的不同有很大差异。尽管对眼外肌收缩性的体外评估将继续揭示有关这组研究不足的肌肉的重要信息，但这些制剂较低的sP(o)值应被视为明显低于其真正潜力。我们得出结论，眼外肌本质上并不比骨骼肌弱。
